1. Field of the Invention
The present invention concerns a device and a method for measuring the composition of the body of a person, said device including a first electronic module which measures the bioelectric impedance of the body and including a current source which delivers a variable electrical signal that passes through the body of the person when the latter is connected to the measuring device.
2. Description of the Prior Art
The composition of the body is measured to assess the nutritional requirement of a person and, as is known per se, by measuring the bioelectric impedance of the body of the person. This provides information on the ratio of lean to fat body mass and the amount of water in the body.
Prior art methods and devices for measuring bioelectric impedance are based on the principle of applying a low alternating current (50 .mu.A to 1500 .mu.A) to the body of the person using two or four electrodes, as shown diagrammatically in FIG. 1. The potential difference that appears between the electrodes is used to calculate the overall impedance Z of the body, which is regarded as a suspension of cells in an electrolytic solution. The global impedance Z of the body is determined from an electrical model of the body, such as the Fricke model shown in FIG. 2, allowing for the frequency f.sub.C of the current used and the resistance to the flow of electrical current due to water and to intracellular and extracellular electrolytes. The respective resistances of said electrolytes are represented by R.sub.I and by R.sub.E in the Fricke model. The calculated bioelectrical impedance also allows for the reactance X.sub.fc of the body membrane, the effect of which is to induce a phase-shift .THETA. between the applied current and the measured voltage. Referring to FIG. 2, R.sub.1, R.sub.2, C.sub.1 and C.sub.2 respectively represent the contact resistances and capacitances of the electrodes. These factors are related by the following equations: EQU Z.sup.2 =R.sup.2 +X.sub.fc.sup.2 EQU X.sub.fc =.vertline.Z.vertline..times.sin .THETA. EQU R=.vertline.Z.vertline..times.cos .THETA.
The locus of impedance of the above equations is a semicircle centered on the abscissa axis (see FIG. 3).
A drawback of devices based on the above method is that a plurality of excitation frequencies must be used to determine the impedance of the body from the FIG. 3 diagram. Furthermore, it has been found that an excessively low frequency is unable to explore the intracellular sector correctly because the capacitance of the membrane represents too high a reactance at this frequency. Moreover, these devices make it necessary to measure the phase-shift .THETA., which is indispensable not only for measuring the impedance Z but also for estimating the resistive part R, which gives the most accurate calculation of lean body mass and total water.
Devices using four electrodes and a single frequency of 50 kHz for which the phase-shift is maximum are known per se. These devices cannot measure specifically the intracellular impedance R.sub.I and the extracellular impedance R.sub.E of the body. They can measure only the global impedance of the body.
The aim of the present invention is to overcome the above drawbacks.